Two United States patent applications, U.S. patent application Ser. No. 11/852,277, entitled, BEAMFORMING WITH NULLING TECHNIQUES FOR WIRELESS COMMUNICATION NETWORKS, filed on Sep. 8, 2007 (hereinafter, “first prior art beamforming technique”), and U.S. patent application Ser. No. 11/963,445, entitled, TECHNIQUES FOR WIRELESS COMMUNICATION NETWORKS EMPLOYING BEAMFORMING, filed on Dec. 21, 2007 (hereinafter, “second prior art beamforming technique”), discuss mechanisms of transmit beamforming with nulling for time-division-duplexing (TDD) and frequency division duplexing (FDD) systems.
The first prior art beamforming technique discusses how to identify mobile stations (MS) that will benefit from the technique. Typically, but not always, such mobile stations are located at the edge of a cell, and are thus known herein as cell edge users. As used herein, cell edge users are those mobile stations residing in a first cell which are served by a first base station, but which may receive interference from a second or more base stations located in a second or more cells of a wireless network, with similar power level compared to the signal power level from the first base station. Transmit beamforming with nulling employed by a base station aims to increase the transmit power to its own cell edge user, while reducing interference to cell edge users served by neighboring base stations. Hence, beamforming with nulling is only effective if all the interfering base stations use it simultaneously. Therefore, it is reasonable to assume that a fraction of channel resources is allocated by the system to serving cell edge users only.
The second prior art beamforming technique describes how base station (BS) transmit beamforming weights may be derived, what pilot support is needed, and the information exchange among neighboring base stations over the backhaul. The second prior art beamforming technique includes a grouping process that will determine which base station/mobile station pairs can share the same resources and which pairs cannot share resources, using specified criteria.
Using these prior art beamforming techniques, a base station with M antennas can null out M−1 interferers, while a mobile station with N antennas can null out N−1 interferers. Hence, the system can null out M+N−2 interfering base station/mobile station links. There can be a maximum of M+N−1 base station/mobile station pairs using the same resources. From simulation, it is found that the system can tolerate more than M+N−2 base station/mobile station pairs. The reason is because it is not necessary to completely eliminate interference. It is sufficient if interference is kept lower than the system thermal noise floor. However, for purposes of illustration, the limit of M+N−2 is used throughout the detailed description.
With multiple cell edge users served by multiple base stations, however, it may be difficult to perform scheduling at each base station such that only base station/mobile station pairs that can share are allocated the same resources.